DVD-RAM is one of the rewritable optical disk standards. An optical disk in a DVD-RAM format having a capacity of 2.6 GB per side is already commercially available. A DVD-RAM disk having an increased capacity of 4.7 GB is currently under development. A recording method for use with a phase change-type optical disk, which is employed in the 4.7 GB DVD-RAM format, is described with reference to FIG. 16.
FIG. 16(a) illustrates data 900 as an example of digital data to be recorded. The data 900 includes a High-level signal 901 of 11T (T denotes a recording clock cycle) and a Low-level signal 902 of 7T, and a High-level signal 903 of 3T. FIG. 16(b) illustrates a waveform of a recording pulse 910 of a laser beam, which corresponds to the data 900. FIG. 16(c) illustrates a recording state of amorphous marks 931 and 932 and a crystal space 933 which are formed on a recording track 934 of an optical disk according to the recording pulse 910.
In order to record the mark 931 and the space 933 which correspond to the data 900, portions of the recording track 934 in which marks are formed are irradiated with a laser beam as the recording pulse 910 which includes: a first pulse 911 having peak power 924; a multi-pulse 912 alternating between the peak power 924 and a third bias power 923 for each period of ½T; a last pulse 913 having the peak power 924; and a cooling pulse 914 having a second bias power 922. Further, the space 933 is irradiated with a laser beam which includes a pulse having a first bias power 921. A waveform of such a recording pulse 910 is generally referred to as “write strategy”. In order to realize proper mark shapes for optical disk recording, the above-described four types of powers and temporal conditions for each of the above-described pulses are individually defined for each optical disk. Further, in order to optimize positions of edges of reproduced waveforms of marks, temporal conditions for a position of each edge of a recording pulse is adaptively controlled for each pattern of marks and spaces (reference document: Japanese Patent No. 2679596). FIGS. 17A and 17B respectively show a first pulse start position table and a last pulse end position table as recording compensation conditions of a 4.7 GB DVD-RAM format. That is, sixteen (4×4=16) types of temporal conditions are defined as start positions of the first pulse 911 for recording the mark 931 shown in FIG. 16(c) according to relationships between a space immediately before a mark and the mark itself with respect to the length, and sixteen (4×4=16) types of temporal conditions are defined as end positions of the last pulse 913 according to relationships between the mark itself and a space immediately after the mark with respect to the length. A procedure for obtaining these conditions is generally referred to as “recording compensation”.
The above-described conventional DVD-RAM write strategy uses the multi-pulse 912 in which a single pulse corresponds to half the width of the recording clock cycle. In order to meet a demand for further increasing an access speed of an optical disk, it is conceivable that a frequency of a recording clock is increased. In this case, the recording clock cycle is inversely decreased, and therefore if a leading characteristic of a laser beam used for recording is not extremely fast, it is not easy to precisely generate the multi-pulse 912. There is a first problem that it is difficult to suppress the cost of elements used in an optical disk apparatus which requires an extremely fast laser drive circuit.
Further, there is a second problem that in the case of detecting and controlling laser power for each mark formation during a recording operation, if there is the multi-pulse 912, which switches fast, in the recording pulse 910, a waveform of the laser beam on the power monitor greatly fluctuates at portions of the multi-pulse 912, thereby requiring a complicated process circuit.
Furthermore, in order to meet a demand for an increase in capacity of the optical disk, it is conceivable that smaller marks are recorded at a high density. There is a third problem that in the case of using a mark edge recording method so as to perform a recording operation at higher density, if the amplitude of a reproduced signal of a shortest mark becomes extremely smaller than those of the other longer marks, the signal is buried in noise so that reproduction error of the optical disk might occur.
Further still, in the case where only the positions of edges of the recording pulse 910 are changed so as to properly position the edges of the reproduced signal, even if recording compensation is achieved, recording power is biased, and therefore there is a possibility that the shapes of the marks might be distorted. In particular, in the case where sizes of front and rear edges of a mark are different, the amplitude of the reproduced signal of the mark are ill-balanced at the front and rear edges of the mark, and therefore errors may occur when digitizing the reproduced signal. There is a fourth problem that even if a multi-value detection means is used for the purpose of reducing errors, satisfactory effects cannot be achieved due to distortion of the shape of the mark.
Further, there are a number of parameters for recording compensation conditions, and therefore a procedure of the recording compensation becomes a complicated and time-consuming task. Therefore, there is a fifth problem that in the case where an automatic recording compensation means is included in the optical disk apparatus, it is difficult to shorten the period of time required to calculate all the parameters.
The present invention solves the above-described first through fifth problems and an objective thereof is to provide: a method for adjusting a recording pulse and write strategy for use in a mark edge recording method, which does not require a multi-pulse having a width thinner than that of a recording clock cycle and can increase an amplitude of a reproduced signal of a shortest mark and reduce distortion in the shape of a reproduced signal of a long mark, thereby reducing the number of recording compensation parameters as compared to conventional methods; and a method for recording/reproducing data on/from a rewritable optical disk using such an adjustment method and write strategy.